I suggest to examine the following online demo which shows a similar scenario:
When you set the RadPane's ContentUrl property you actually create an IFRAME element in it and the pane behaves exactly as an IFRAME. This IFRAME is needed to be targeted by the hyperlink. In your case, the initial IFRAME is not created and therefore the needed url cannot be loaded.
This being said in order to use this approach, you should initially set some ContentUrl to the target RadPane (e.g about:blank or a custom start page) in order to force it to create an IFRAME element.
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